At the White House this afternoon, President Trump defended his decision to send as many as 15,000 active duty troops to the border with Mexico. It would be the biggest domestic deployment of active duty soldiers in modern history. The president justified this mobilization by pointing to a caravan of Central American migrants close to a thousand miles from the U.S. border making its way north.

NPR's David Welna joins us now. Hi, David.

DAVID WELNA, BYLINE: Hey, Ari.

SHAPIRO: We've heard the president talk a lot about this migrant caravan ever since it crossed into Mexico from Guatemala 11 days ago. Did he say anything new today?

WELNA: He did. He said that those caught at the border will no longer be released until they have a court hearing, which has been the practice because there haven't been enough places to detain these migrants. And he said the children will be held with their parents in tent cities that the military will be building. He also said people will be able to apply for asylum only at legal points of entry along the border. Right now they can do so anywhere. And when he asked - he was asked if the military will be shooting people. He said he hoped not.

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PRESIDENT DONALD TRUMP: But I will tell you this. Anybody throwing stones, rocks like they did to Mexico and the Mexican military, Mexican police where they badly hurt police and soldiers of Mexico - we will consider that a firearm.

WELNA: And so that would appear to be a rationale for shooting stone throwers.

SHAPIRO: In April, Trump ordered a couple thousand National Guard reservists to the Mexican border. Fifteen thousand active duty soldiers is a much bigger presence. Why does he say this is needed now?

WELNA: Well, a reporter asked him if this was all political given its timing right before the midterm elections, and Trump pushed back on that.

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TRUMP: There's nothing political about a caravan of thousands of people and now others forming pouring up into our country. We have no idea who they are. All we know is they're pretty tough people when they can blast through the Mexican military and Mexican police. They're pretty tough people.

WELNA: Trump has also asserted that there are gang members and tough criminals as well as Middle Easterners among the 3,000 or so people that the Department of Homeland Security estimates are in this group of migrants, but he's offered no evidence to back up those claims.

SHAPIRO: The defense secretary authorized this movement of troops to the border. What does he have to say about sending this many people?

WELNA: Well, Jim Mattis has fallen in step with Trump on this deployment. He was asked yesterday at the Pentagon by a reporter if this was a stunt, and his response was, we don't do stunts in this department.

SHAPIRO: How long are the troops likely to stay there?

WELNA: Well, right now the plan is to keep the troops there only until the middle of December. It's not clear whether the folks in this caravan could even reach the U.S. border by then. They still have about 900 miles to go. And for now, it looks like they'll be walking that distance if they indeed even make it that far.